Hippocampal neurons have widely been used in the field of the neurophysiology as central nerve cells that can be cultured in laboratories. It has been known that a significant number of the neurons die in one week from the start of the culture when the cells are primarily cultured alone. As a method for long-term culture of hippocampal neurons, co-culture of the cells with glial cells (gliacytes that fill spaces between neurons and their neurites) is known. However, since the culture system is not a monoculture system, it is not suitable for researches on auxotrophy of hippocampal neurons alone, neuronal responses to polypeptide neurotrophic factors and the like. As a method for culturing hippocampal neurons in the absence of glial cells, a method is known wherein the neurons are cultured in the presence of all non-essential amino acids. However, survival time of the cells and number of survived cells are significantly lower than those attained by the co-culture with glial cells.
In primary culture system of hippocampal neurons, it has also been known that long term survival of neurons can be achieved by the addition of culture supernatant of glial cells (astrocyte conditioned medium, ACM). A method for such culture has been established by Goslin et al. (Goslin, K. and Banker, G., "Culturing Nerve Cells", Ed. by Banker, G. et al., p.251-278, The MIP Press, England). However, it has not been revealed which substance in the culture supernatant enhances survival of the neurons.
It has also been known that L-serine acts as an important factor for morphodifferentiation of fowl ganglions which are peripheral nerve cells (Savoca, R., Ziegler, U. and Sonderegger, P., Journal of Neuroscience Methods, 61, pp.159-167, 1995). However, the action of L-serine disclosed in the publication is mainly focused on the morphogenesis of neurons, and the publication neither teaches nor suggests whether L-serine may have any action on the survival of neurons. Moreover, the cells used in the experimental system were peripheral nerve cells, which are totally different from central nerve cells such as hippocampal neurons in generation and functions. Therefore, action of L-serine on central nerve cells is not taught by the publication